1. Field of the Invention
The present invention relates to a method for removing crystal regions from silicon wafers which are damaged as a consequence of mechanical machining of the silicon wafers.
2. The Prior Art
This kind of damage to the crystal structure inevitably occurs in the production of silicon wafers. Even as a result of the cutting of individual silicon wafers from a pulled monocrystal with the aid of a sawing tool, for example a wire saw or an annular saw, the subsurface crystal regions of the resulting silicon wafers are damaged down to a depth of approximately 80 .mu.m. This damage resulting from sawing is, as a rule, reduced by a mechanical surface treatment of the silicon wafers, for example by lapping or grinding the wafers. Lapped or ground silicon wafers therefore only have damage in the crystal structure which extends from the wafer surface down to a depth of approximately 35 .mu.m into the interior of the wafer. This damage, too, has to be removed completely if it is planned to integrate electronic circuits on the surface of the silicon wafer at a later point in time.
A known method of removing mechanically damaged crystal regions from silicon wafers is based on anisotropic chemical etching of the silicon wafers in an aqueous bath containing, for example, potassium hydroxide and/or sodium hydroxide at temperatures from 100.degree. C. to 120.degree. C. A particular advantage of this method, which is simple to carry out, is that it does not adversely affect the wafer shape and the flatness of the wafer side faces, which are determined by mechanical machining (L. D. Dyer, G. J. Grant, C. M. Tripton, A. E. Stephens, Electrochem. Soc. Symp. Ser., pp. 296, June, 1989).
On the other hand, etching under alkaline conditions is known for the fact that it leaves behind as contaminants foreign metals, which partly originate from the alkali metal hydroxide used, on the wafer surfaces and that such foreign metals diffuse into the interior of the wafers from the etched surface because of the prevailing, relatively high temperatures. Since foreign metal atoms severely interfere with the operation of the electronic components, special care has to be taken in the production of the silicon wafers to ensure that the contamination due to foreign metals is as low as possible. Foreign metals which diffuse into the interior of the wafers during the alkaline etching are therefore a particular problem because they can no longer be removed even by a subsequent cleaning of the wafer surfaces. An improvement in the procedure by using purer alkali metal hydroxides has hitherto been thwarted by the high industrial and economic cost associated with providing such reagents.